Conventionally, an electro-photographic type image forming apparatus such as a laser printer and an LED printer is known in the art. The image forming apparatus includes a drum unit. The drum unit includes a plurality of photosensitive drums. A plurality of developing cartridges is detachably attachable to the drum unit. When a developing cartridge is attached to the drum unit, a developing roller provided in the developing cartridge contacts a corresponding photosensitive drum provided in the drum unit.
Japanese Patent Application Publication No. 2010-128336 discloses such an image forming apparatus including a drum unit.